Catalysts used for exhaust gas purification in an internal combustion engine have an oxygen occlusion capability for occluding oxygen in them. When the air-fuel ratio of an exhaust gas flowing into a catalyst is lean, the catalyst occludes oxygen in the gas. When, on the other hand, the air-fuel ratio of the exhaust gas flowing into the catalyst is rich, the catalyst releases the occluded oxygen into the gas. Therefore, when the exhaust gas has a lean air-fuel ratio and contains a relatively large amount of NOx as compared with HC and CO, the catalyst occludes oxygen to reduce NOx. When, on the other hand, the exhaust gas has a rich air-fuel ratio and contains a relatively large amount of HC and CO, the catalyst releases oxygen to oxidize HC and CO.
However, if the air-fuel ratio of the exhaust gas flowing into the catalyst continues to deviate toward the lean side, the oxygen occluded by the catalyst reaches saturation before long so that NOx cannot be purified. If, in contrast, the air-fuel ratio continues to deviate toward the rich side, the oxygen occluded by the catalyst is depleted before long so that HC and CO cannot be purified. Under such circumstances, conventional internal combustion engines exercise fuel injection amount feedback control in accordance with an oxygen sensor output value to ensure that the oxygen occluded by a catalyst is maintained in an appropriate state.
The oxygen occluded by a catalyst can be monitored when an oxygen sensor is installed downstream of the catalyst. When the oxygen in the catalyst is saturated, the output value generated from the oxygen sensor changes from rich to lean. When, in contrast, the oxygen in the catalyst is depleted, the output value generated from the oxygen sensor changes from lean to rich. Therefore, when the oxygen sensor's output value is fed back to the fuel injection amount to increase or decrease the fuel injection amount in accordance with changes in the oxygen sensor's output value, the oxygen occluded by the catalyst can be maintained in an appropriate state.
Further, it is known that the catalyst's oxygen occlusion capability can be maintained high when the catalyst's noble metal is activated by repeatedly occluding and releasing oxygen. When the catalyst's oxygen occlusion capability is high, oxygen can be occluded or released to purify NOx, HC, and CO in the exhaust gas with high efficiency even if the air-fuel ratio of the exhaust gas is significantly varied from a stoichiometric air-fuel ratio or oscillating with large amplitude. According to fuel injection amount feedback control that is exercised in accordance with the oxygen sensor's output value, the catalyst can repeatedly occlude and release oxygen as the air-fuel ratio of the exhaust gas oscillates around the stoichiometric air-fuel ratio.
Air-fuel ratio control methods for making effective use of a catalyst's oxygen occlusion capability are described in the patent documents enumerated below:    Patent Document 1: JP-A-2002-115590    Patent Document 2: JP-A-2005-188330    Patent Document 3: JP-A-1998-246139